Add support for checking minimum PHP version in XML update stream

This introduces a <php_minimum> tag which should indicate the minimum supported PHP version of an extension
Optimize version_compare check, thanks @dongilbert
Add message if an update is available but unsupported
Restructure PHP version check
Merge remote-tracking branch 'joomla/staging' into phpUpdateLib
Filter images in JFormFieldImageList

Filter attribute has to be set before field is setup. Otherwise filter would not be used and there are all files from folder in a drop-down
Update imagelist.php

Corrected mistake
Update imagelist.php
[imp] [33196] Add "showon" feature to component configuration.
Fix #2847
[CODE STYLE] Added missing newline
[CODE STYLE] Corrected indentation
[CODE STYLE] Improved indentation/newlines
[CODE STYLE] Corrected indentation
[CODE STYLE] Removed extra space
[CODE STYLE] Added missing newlines
[CODE STYLE] Added missing whitespace
[CODE STYLE] Added missing whitespace
[CODE STYLE] Added missing whitespace
[CODE STYLE] Added missing whitespace
[CODE STYLE] Added missing whitespace